The physical, cognate interactions of helper T cells (Th) with antigen- presenting B cells in essential for the development of humoral immunity. It is the binding of specific surface molecules on interacting Th and B cells that initiates Th-dependent humoral immune responses. Studies presented in this proposal provide insights into the molecular basis for Th-B interactions and the biochemical signalling that occurs between Th and B cells. Based on these preliminary studies. a comprehensive study is proposed to elucidate the underlying mechanisms responsible for cognate activation of B cells by Th. During the course of Th-B interaction, the molecular requirements for conjugate formation change. Initially, conjugate formation between resting, antigen-specific Th and B cells in Class II-restricted and antigen-specific (recognition phase of T cell help). This phase of T Cell help results in Th activation, but does not induce B cells to enter the cell cycle. Later, only after Th activation has occurred, the activated Th reciprocally triggers B cells to enter the G1 phase of the cell cycle and become responsive to lymphokines (effector phase of T cell help). Triggering of B cells by activated Th is neither class II MHC-restricted nor antigen-specific. Our data suggest that: 1) the binding of CD4 molecules on activated Th to monomorphic domains of class II MHC molecules on B cells and 2) the de novo expression of new Th cell surface proteins, induces B cells to enter the G1 phase of the cell cycle. Activated Th, but not resting Th, that were fixed with paraformaldehyde retaining their ability to induce B cells to enter the G1 phase of the cell cycler. This provided proof that cell-cell contact, and not lymphokines, induced B cell contact were measured. Increase B cell RNA synthesis and increased intracellular levels of cAMP have been shown to result from Th-B cell contact. The use of synthetically inactive, fixed Th in combination with purified lymphokines has allowed the unambiguous identification of the lymphokines required for the sequential progression of Th-activated B cells through the cell cycle. IL4 and another, as yet unidentified lymphokine, ILX, were essential for Th-activated B cells to progress into S phase. Using this system as a means to assay for ILX activity, the biochemical characterization and molecular cloning of ILX will be pursued. Our findings that activated Th induce B cell activation in an unrestricted manner and that fixed Th mimic the action of viable Th provides the opportunity to use fixed Th as a physiologically relevant reagent to study how Th activate B cells. We propose to: 1) identify the Th and B cell surface molecules that are required for Th-B conjugate formation and B cell activation, 2) identify the B cell second messenger systems that are activated as a result of Th B cell contact and 3) characterize lymphokine(s) that support the progression of Th-activated B cells into the S phase of the cell cycle. This comprehensive analysis of the effector phase of T cell help will provide important insights into the regulation of humoral immunity.